[Paper Review] Cosmic-ray Antimatter

Kfir Blum, Ryosuke Sato+1 more|arXiv (Cornell University)|Sep 19, 2017

Dark Matter and Cosmic Phenomena2 references3 citations

TL;DR

This paper provides a theoretical framework for interpreting cosmic-ray antimatter measurements from space experiments like AMS-02, showing that secondary production from cosmic-ray collisions with interstellar gas explains the observed fluxes of antiprotons and positrons. It demonstrates that composite antinuclei such as $\bar{d}$ and $^{3}\overline{He}$ can be predicted with high precision using calibrated propagation models, enabling future detection to probe new physics or constrain astrophysical sources.

ABSTRACT

In recent years, space-born experiments have delivered new measurements of high energy cosmic-ray (CR) $\bar p$ and $e^+$. In addition, unprecedented sensitivity to CR composite anti-nuclei anti-d and anti-He is expected to be achieved in the near future. We report on the theoretical interpretation of these measurements. While CR antimatter is a promising discovery tool for new physics or exotic astrophysical phenomena, an irreducible background arises from secondary production by primary CR collisions with interstellar matter. Understanding this irreducible background or constraining it from first principles is an interesting challenge. We review the attempt to obtain such understanding and apply it to CR $\bar p,\, e^+,$ anti-d and anti-He. Based on state of the art Galactic cosmic ray measurements, dominated currently by the AMS-02 experiment, we show that: (i) CR $\bar p$ most likely come from CR-gas collisions; (ii) $e^+$ data is consistent with, and suggestive of the same secondary astrophysical production mechanism responsible for $\bar p$ and dominated by proton-proton collisions. In addition, based on recent accelerator analyses we show that the flux of secondary high energy anti-He may be observable with a few years exposure of AMS-02. We highlight key open questions, as well as the role played by recent and upcoming space and accelerator data in clarifying the origins of CR antimatter.

Motivation & Objective

• To interpret recent high-precision cosmic-ray antimatter measurements from space-based experiments like AMS-02.
• To establish a model-independent method for predicting secondary antimatter fluxes using stable secondary nuclei as calibration tools.
• To assess the role of secondary production in explaining the observed fluxes of $\bar{p}$, $e^+$, $\bar{d}$, and $^{3}\overline{He}$.
• To evaluate the sensitivity of future experiments to detect secondary $^{3}\overline{He}$ within a few years of AMS-02 exposure.
• To clarify the implications of $e^+$ data for cosmic-ray propagation and the potential for dark matter or pulsar signals.

Proposed method

• Uses the B/C ratio as a proxy to calibrate cosmic-ray propagation models, reducing dependence on phenomenological assumptions.
• Applies the concept of 'grammage' to compute the total interstellar matter traversed by cosmic rays, enabling parameter-free predictions for antimatter fluxes.
• Derives the $e^+$ loss suppression factor $f_{e^+}$ in both leaky-box and diffusion models to account for radiative energy loss during propagation.
• Employs rigidity-dependent escape times $t_{\rm esc}(\mathcal{R})$ to relate propagation time to column density $X_{\rm esc}(\mathcal{R})$ via $X_{\rm esc} \propto t_{\rm esc}$.
• Uses accelerator data to constrain the secondary production cross-sections for $^{3}\overline{He}$, enabling flux predictions.
• Compares model predictions for $\bar{p}/p$ and $e^+$ with AMS-02 data to test consistency with secondary production.

Experimental results

Research questions

• RQ1 Can the observed flux of cosmic-ray antiprotons be explained by secondary production from proton-proton collisions with interstellar gas?
• RQ2 Is the observed positron excess in cosmic rays consistent with secondary production, or does it require primary sources like pulsars or dark matter?
• RQ3 To what extent can the flux of secondary $^{3}\overline{He}$ be measured with AMS-02, and when?
• RQ4 How robust are antimatter flux predictions when calibrated using stable secondary nuclei like B/C?
• RQ5 What is the role of radiative energy loss in suppressing $e^+$ fluxes, and how does it affect propagation model interpretations?

Key findings

• The observed antiproton flux is consistent with secondary production from proton-proton collisions, with no significant excess requiring new physics.
• Positron data is consistent with secondary production via proton-proton collisions, suggesting no need for primary sources like pulsars or dark matter to explain the flux.
• The secondary $^{3}\overline{He}$ flux is predicted to be observable with a few years of AMS-02 exposure, based on recent accelerator data.
• The $\bar{p}/p$ ratio can be predicted with high precision using the B/C ratio as a calibration tool, minimizing propagation model uncertainties.
• The $e^+$ loss suppression factor $f_{e^+}$ depends on the model, but both leaky-box and diffusion models show a common scaling $f_{e^+} \propto \sqrt{t_{\rm cool}/t_{\rm esc}}$ in the strong loss regime.
• The grammage concept allows for a parameter-free prediction of antimatter fluxes, decoupling the result from detailed propagation assumptions.